1. Field of the Invention
The present invention relates to a resin-coated aluminum seamless can body for the contents of beverages such as carbonic beverages, beer, juice, alcoholic beverages and water. More specifically, the invention relates to a resin-coated aluminum seamless can body which features superior resistance against cracks in the can wall during distribution, and against flange cracking.
2. Prior Art
Conventionally, draw-ironed aluminum cans that are widely used for beverage cans and such are comprised of a can barrel, which is continuously joined to the bottom plate of the can, having been produced by punching an aluminum plate into a disk-shape to make a blank, and then decreasing the thickness of a plate of the can wall by drawing or redraw-ironing, and thus narrowing the upper portion of the can wall, so as to mount a lid on the opening (the neck in portion). This method of forming cans is commonly used because it decreases the thickness of a plate of the can wall itself, without decreasing the thickness of a plate of the bottom of the can, which is necessary for resistance against pressure, as well as significantly reduces the material as compared to conventional three-pieced cans.
Furthermore, in recent years, the thickness of a plate of the can wall has been decreasing as a cost-cutting policy in producing cans by minimizing the amount of material used for the cans. For instance, as for the cylindrical can side wall, the drawing process is followed by redraw-ironing, which decreases the thickness of the plate by a ratio of as much as 60 to 70 percent, using an aluminum plate wherein the thickness of a plate is 0.3 mm before after forming, and by a DI process that decreases the thickness of a plate of the side wall of the can by around 0.10 mm. Moreover, research and development on decreasing the thickness of a plate of the can wall continues to be conducted.
Therefore, because the carbonic gas itself causes internal pressure on the can wall, aluminum drawn and ironed cans with a significantly decreased thickness of a plate of the can wall are often applied to cans containing beer and carbonated beverages. Furthermore, in the case of cans with contents such as tea beverages, which do not cause internal pressure on the can wall, the cans are filled with liquid nitrogen gas (pressurized can).
As for these beverage cans filled with contents in draw-ironed aluminum cans with a decreased thickness of a plate of the side wall of the can, because of the extreme decrease in the thickness of a plate of the side wall of the can, problems during the process of distribution, such as cracks in the side wall of the aluminum plate and the contents spurting out when the can is accidentally dropped, while in a carton case or by itself, or when the can is dented (for example, when it hits against the corner of a desk or when there is any kind of small protrusion at the place where it is dropped), need to be improved upon. That is, after the can is filled with its contents and after the lid has been forced on, during the process of distribution or handling, if a protrusion or so forth from the outside of the can is pressed against the can wall, or if the can experiences any shock from being dropped, in the case of cans with a very thin side wall, cracks have occurred in the side wall of the can (in the present invention these may be called “cracks in the can wall”).
Particularly, in case of a pressurized can, such as carbonated beverages, where the inside of the can is pressurized, a small hole immediately develops into the origin of a crack, and there is a risk that all of the contents of the beverage can will leak out. In Patent Reference 1 it has been proposed that, in order to improve the strength of beverage cans, specific aluminum alloys have been used as a material for the components of the can, and moreover the elongation break of the can has been improved by means of heat treatment.    Patent Reference 1: Japanese Unexamined Patent Publication (Kokai) No. 199273/1996